Ammonium phosphate solutions which contain a high percentage of the phosphorus present as polyphosphates are highly desirable because the polyphosphates are subsequently more soluble than orthophosphate. When the phosphate is derived from wet-process phosphoric acid, which constitutes the majority of commercial phosphoric acid, the polyphosphates also function to chelate the congeneric, metallic impurities of the wet-process phosphoric acid and avoid their precipitation in the ammonium phosphate solution. A high concentration of polyphosphates for this purpose is also desired to avoid any instability upon storage of such solutions that could be caused by the hydrolysis of the polyphosphate to levels below its effective concentration for preventing precipitation of the metallic impurities.
The formation of polyphosphoric acid or polyphosphates requires the expenditure of a considerable amount of energy for the evaporative dehydration of the phosphoric acid. It has been suggested that the exothermic heat of neutralization of phosphoric acid with ammonia be utilized as a source of energy for the molecular dehydration of the acid; see German Patent 6321. While this technique is operative for laboratory demonstrations, heretofor it has not been utilized in an entirely successful commercial installation. A large number of prior attempts have been made and many of these have concentrated on the reaction of ammonia and phosphoric acid in a tubular reactor using a continuous flow system. Typical of these are U.S. Pat. Nos. 2,902,342; 3,419,379; 3,420,624; 3,464,808; and 3,649,175.
The tubular reactor is ideally suited for this reaction since it provides a minimal residence time of the reactants and products and provides sufficient turbulence for adequate mixing of the reactants, completion of the reaction and dehydration of the orthophosphate. Phosphoric acid, however, and, in particular, wet-process phosphoric acid, is not readily amenable to such processing. The high temperatures involved in the evaporative neutralization of the acid can cause formation of insoluble precipitates with the metallic impurities. These precipitates occur because the temperatures are often sufficient to form minute amounts of cyclic or metaphosphates. The latter react with the metallic impurities to form refractory precipitates which coat the inside surfaces of the reactor, often plugging it and requiring discontinuance of the reaction after a few hours of operation. Attempts to lessen the degree of this precipitation generally involve limiting the reaction to temperatures less than the maximum which could be attainable.
It is, therefore, an object of this invention to provide a process for the production of ammonium polyphosphates which will achieve the maximum conversion to polyphosphates.
It is, furthermore, an object of this invention to provide such a process which utilizes substantially all the exothermic heat of reaction between ammonia and phosphoric acid to concentrate the acid and effect molecular dehydration of the phosphates.
It is a further object of the invention to operate such a process at the maximum temperature that can be obtained and to operate the process under substantially abiatic conditions.
It is also an object of the invention to provide a reactor which can be used in the aforestated process for prolonged periods of time without requiring repair, maintenance and cleaning.
Other and related objects will be apparent from the following discussion of the invention.